In the past we have isolated mutants of the fruit fly, Drosophila melanogaster, that render the organism relatively resistant to the effects of the general anesthetic, halothane. In the present work, we have refined the genetic and physiological characterization of two of these mutants. The genetics shows that the two mutants define a single locus and the physiology implies that the mutants have an altered neuromuscular system. Studies of the response of several mutants to anesthetics other than halothane indicate that each mutant affects a different anesthetic target.